Red iron oxide pigments are used in the building material, plastics material and lacquer industries. Color-pure red iron oxide pigments with a distinct red tint are particularly desired for use in lacquer systems. The tint of the pigment is defined in the lacquer according to DIN 6174 (equivalent ISO/DIN 7724, 1-3 drafts). The saturation value C* determined in the lacquer is valid as a measure for the color purity of the red iron oxide pigment, and the value a* determined in the lacquer is valid as a measure for the red tint.
When incorporating a red iron oxide pigment into various binder systems, it is observed that the saturation value C* and the value a* of the pigments decrease in numerous binder systems. The pigment grindings in such binder systems are thus substantially more unsaturated.
The pigments lose color purity in numerous binder systems independently and regardless of the process of production of red iron oxide pigments (e.g. by thorough roasting of iron sulphate or by poor annealing of .alpha.-FeOOH or Fe.sub.3 O.sub.4, which have been obtained by the precipitation route or aniline route or by production of direct red; Ullmann, volume 18, p. 601-603, 4th edition, Weinheim, 1979).
The object of the present invention is thus to provide red iron oxide pigments which do not suffer from the disadvantages described above.